Clarified in the docs how to get to the serialized bytes.

This was a frequent source of confusion, since in all implementations
the data doesn't start at offset 0 in the buffer.

Change-Id: I045966e65928e9acd9def84e215914ecb5510653

docs/source/Tutorial.md

@@ -1161,12 +1161,23 @@ like so:
 ~~~{.java}
   // This must be called after `finish()`.
   java.nio.ByteBuffer buf = builder.dataBuffer();
+  // The data in this ByteBuffer does NOT start at 0, but at buf.position().
+  // The number of bytes is buf.remaining().
+
+  // Alternatively this copies the above data out of the ByteBuffer for you:
+  bytes[] buf = builder.sizedByteArray();
 ~~~
 </div>
 <div class="language-csharp">
 ~~~{.cs}
   // This must be called after `Finish()`.
   var buf = builder.DataBuffer; // Of type `FlatBuffers.ByteBuffer`.
+  // The data in this ByteBuffer does NOT start at 0, but at buf.Position.
+  // The end of the data is marked by buf.Length, so the size is
+  // buf.Length - buf.Position.
+
+  // Alternatively this copies the above data out of the ByteBuffer for you:
+  bytes[] buf = builder.SizedByteArray();
 ~~~
 </div>
 <div class="language-go">
@@ -1184,13 +1195,16 @@ like so:
 <div class="language-javascript">
 ~~~{.js}
   // This must be called after `finish()`.
-  var buf = builder.dataBuffer(); // Of type `flatbuffers.ByteBuffer`.
+  var buf = builder.asUint8Array(); // Of type `Uint8Array`.
 ~~~
 </div>
 <div class="language-php">
 ~~~{.php}
   // This must be called after `finish()`.
   $buf = $builder->dataBuffer(); // Of type `Google\FlatBuffers\ByteBuffer`
+  // The data in this ByteBuffer does NOT start at 0, but at buf->getPosition().
+  // The end of the data is marked by buf->capacity(), so the size is
+  // buf->capacity() - buf->getPosition().
 ~~~
 </div>
 <div class="language-c">